Notizen 1145 Note on the Influence of External Heavy-Atom
Perturbers on the Phosphorescence Spectrum of Triphenylene
M. Zander
Rütgerswerke AG, Castrop-Rauxel Z. Naturforsch. 39a, 1145-1146 (1984);
received September 21. 1984
Spectral changes in symmetry-forbidden phosphores
cence spectra observed in the presence of external heavy- atom perturbers may have quite different causes depend
ing on the chemical nature of the perturber. This is exem
plified using triphenylene as the phosphorescent com
pound and methyl iodide and silver Perchlorate respective
ly as the perturber. Intensification of the 0-0 band of the symmetry-forbidden phosphorescence spectrum of tri
phenylene by silver Perchlorate is assumed to result from symmetry-reduction of the hydrocarbon by ground-state complex formation with silver Perchlorate.
The radiative T, —<■ S0 transition of triphenylene is both spin- and symmetry-forbidden [1]; corre
spondingly, the intensity of the 0-0 band (427 nm) in the phosphorescence spectrum of triphenylene is very low (Fig. 1, curve a; ethanol, 77 K). External heavy-atom perturbers, e.g. methyl iodide, enhance strongly the intensity of the 0-0 band [2, 3]. This is due to a second-order mixing of perturber singlet- state character into the T, state of the phosphores
cent molecule [4]. As associative forces between the perturber and the aromatic hydrocarbon are negli
gible perturber and aromatic molecules are random
ly distributed in the rigid matrix. On the contrary.
silver salts, e.g. AgC104. as external heavy-atom perturbers form ground-state complexes with aro
matic hydrocarbons [5] that are well-defined regard
ing stoichiometry and geometry. In many cases these ground-state complexes are phosphorescent [6. 7. 8], The subject of the present communication is a comparative study of the influence of methyl iodide and AgC104 respectively as external heavy- atom perturbers on the phosphorescence spectrum of triphenylene in ethanol at 77 K.
As a measure for the external heavy-atom effect the ratio of intensities of the 0-0 band (427 nm) to the band at 460 nm which is the most intense band in the unperturbed spectrum was used (Fig. 1, curve a). This ratio (R\= U ii^ m ) is 0.1 in the un
perturbed spectrum. The dependence of R\ on the perturber concentration (CH3J, AgC104) is shown in Figure 2. While the maximum /?j value is 1.5 if CH3J is used as the perturber it is 0.45 with AgC104. The concentration limit of perturber above which Rx becomes constant is ^ 2 M for CH3J and
^ 0.75 M for AgC104. Also in pure CH3J as a matrix R] amounts to 1.5.
Apart from the different R\ values the phospho
rescence spectra of triphenylene in the presence of CH3J and AgC104 respectively are rather similar (Fig. 1, curves b and c).
However, the phosphorescence decay behaviour of triphenylene depends strongly on the type of perturber. As in many similar cases [3, 9] the decay curve is multi-exponential if CH3J is used as the perturber, e.g. 4 time constants could be resolved at a CH3J concentration of 1.8 M (0.6, 1.7, 4.2 and 15.6 sec the latter being the phosphorescence life-
X [ nm ]
Fig. 1. Phosphorescence spectra of triphenyl
ene (2- 10_4M) in ethanol at 77 K: a) in the absence of an external heavy-atom perturber, b) in the presence of methyl iodide (1.8 M), c) in the presence of silver Perchlorate (1 M).
(All spectra were normalized to the same heigth of the most intense band.)
Reprint requests to Professor Dr. M. Zander. Rütgerswerke AG. D-4620 Castrop-Rauxel.
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1146 Notizen
Fig. 2. Dependence of the ratio (/?]) of the intensities of the 0-0 band (427 nm) to the band at 460 nm of the tri- phenylene phosphorescence spectrum on the perturber concentration (CH3J. AgC104) (Ethanol. 77 K).
time of the unperturbed molecule). On the contrary, the phosphorescence decay of triphenylene is mono- exponential in the presence of AgC104 at a concen
tration ^ 0.75 M (vide supra) exhibiting the phos
phorescence lifetime (20 msec) of the triphenylene/
AgC104 ground-state complex. Below this concen
tration the phosphorescence lifetime of the complex and the unperturbed triphenylene is observed. An analogous phosphorescence decay behaviour has been found in other aromatic donor molecule/silver salt systems [8], It can be assumed that as in similar cases the Ag+ ion induced increase of the rate con
stant kpT of the radiative deactivation of the tri
phenylene triplet state is larger compared to the increase of FGX of the radiationless deactivation [7],
For perturber molecules like methyl iodide for which associative forces between perturber and phosphorescent compound can be neglected, the exchange mechanism is well established as being responsible for the intensification of the 0-0 band in
symmetry-forbidden phosphorescence transitions (vide supra. [4. 10]). Since the effect is purely elec
tronic in nature, i.e. no vibronic coupling is in
volved, the extent to which spectral changes occur resembles that of changes in the phosphorescence lifetime. However, in the triphenylene/AgC104 sys
tem the change in phosphorescence lifetime is much larger compared to the triphenylene/CH3J system but. on the other hand, the intensification of the 0-0 band is lower. Thus, another explanation for the spectral changes observed in the triphenylene/
AgC104 system seems to be more likely.
The experimental results are consistent with the assumption that in the triphenylene/AgC104 system a phosphorescent ground-state complex is formed.
In this complex the silver ion acts like a substituent which reduces the symmetry of the aromatic hydro
carbon and thus relaxes the symmetry-forbidden- ness of the phosphorescence transition. Relaxation of the parity selection rule by substituents linked to centrally symmetric molecules whose phosphores
cence transition is symmetry-forbidden is known in other cases, e.g. the very weak 0-0 band in the phos
phorescence spectrum of coronene is significantly enhanced in the phosphorescence spectrum of methyl-coronene [11].
Experimental
The experiments were done on an Amino-Keirs spectrophosphorimeter using an oscillograph Tek
tronix 5403 for the phosphorescence lifetime mea
surements.
The triphenylene concentration used was always 2 - IO"4 M.
The results obtained proved independent of the excitation wavelength.
Experimental assistance of Mr. K. Bullik is grate
fully acknowledged.
[1] J. Czekalla and K. J. Mager. Z. Elektrochem. 66, 65 (1962); F. Dörr and H. Gropper, ibid. 67, 193 (1963).
[2] M. Zander, Naturwiss. 52,559 (1965).
[3] M. Zander. Fresenius Z. analyt. Chem. 226, 251 (1967).
[4] G. G. Giachino and D. R. Kearns. J. Chem. Phys.
53,3886 (1970).
[5] L. J. Andrews and R. M. Keefer, J. Amer. Chem. Soc.
71, 3644 (1949); R. E. Kofahl and H. J. Lucas, ibid.
76,3931 (1954).
[6] G. D. Boutilier and J. D. Winefordner, Anal. Chem.
51,1391 (1979).
[7] M. Zander, Z. Naturforsch. 37 a, 1348 (1982).
[8] M. Zander. Z. Naturforsch., in press.
[9] S. P. McGlynn. M. J. Reynolds, G. W. Daigre, and N. D. Christodouleas. J. Phys. Chem. 66,2499 (1962).
[10] J. Najbar. J. B. Birks, and T. D. S. Hamilton, Chem.
Phys. 23,281 (1977).
[11] M. Zander. Phosphorimetry, Academic Press. New York 1968. p. 78.
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